Hypothermia

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Chapter 67 Hypothermia

Howard M. Corneli, MD

1 Why are children more prone to hypothermia than adults?

Children have larger body surface–to–mass ratios.

They may have less insulation, faster metabolism, and smaller energy reserves.

They may be more at risk for medical illness, trauma, or environmental exposure.

2 Does hypothermia require severe cold exposure?

No. Unfortunately, clinicians may suspect hypothermia only when severe environmental exposure is evident. Any immersion in warm or cold water, or any major illness or trauma, may precipitate hypothermia. It is also seen in conditions as diverse as sepsis, hypoglycemia, drug ingestion, and child abuse, any of which may be occult.

Children are more at risk than adults for iatrogenic hypothermia due to rescue, transport, and resuscitation.

3 Why is hypothermia overlooked in pediatric patients in the emergency department (ED)?

Hypothermia victims only appear cold in the mild range of hypothermia. In deeper hypothermia, signs such as shivering, pallor, cyanosis, and agitation are replaced by flushing, edema, muscular rigidity, and decreased mental status. Standard clinical thermometers only measure down to 94° F (34° C).

4 What are some risk factors for hypothermia?

Trauma	Rescue
Severe illness	Transport
Immersion or submersion	Resuscitation
Exposure to wind or cold air	Intoxication, especially with:
Central nervous system illness or injury	Alcohol
Hypothalamic dysfunction	Barbiturates
Endocrine impairment	Phenothiazines
Metabolic impairment	Burns and weeping dermatoses
Iatrogenic causes	Child abuse (e.g., cold water baths)

5 What clinical clues should prompt measurement of core temperature?

Any of the above risk factors (including, but not limited to, any critical care patient)

Decreased mental status, respiration, or circulation without other obvious cause

A J (Osborne) wave on electrocardiogram (Fig. 67-1) is diagnostic when present, but is seen in only about 10–80% of cases in various series.

Figure 67-1 EKG showing Osborne or J wave.

Nolan J, Soar J: Images in resuscitation: The ECG in hypothermia. Resuscitation 64:133, 2005.

6 How should temperatures be taken to detect hypothermia?

A low-reading thermometer must be used. Oral and axillary temperatures are unreliable. Rectal temperatures are subject to damping, artifact, and time lags up to 1 hour. If used, rectal temperatures must be taken deep in the rectum for at least several minutes. The reliability of tympanic temperatures in the setting of significant hypothermia has not been established. Temperature probes in the bladder, esophagus, or nasopharynx are typically used to provide core temperatures of the trunk.

Danzl DF, Pozos RS: Accidental hypothermia. N Engl J Med 331:1756, 1994.

7 Define mild, moderate, and severe hypothermia

Mild hypothermia: Temperature ranges from approximately 32° to 35° C. In this range, the body attempts to combat heat loss by shivering, increasing metabolism and vasoconstriction.

Moderate hypothermia: Temperature spans the range from 28° to 32° C. In this range, compensatory mechanisms begin to fail, and mental status may be altered.

Severe hypothermia: Temperature (< 28° C) leads to failed thermoregulation, metabolic shutdown, and paradoxical vasodilatation accompanied by hypovolemia, decreased perfusion, and stupor or coma.

Corneli HM: Hot topics in cold medicine: Controversies in accidental hypothermia. Clin Pediatr Emerg Med 2:179–191, 2001.

8 Describe some landmarks for converting between Celsius and Fahrenheit

At the upper end of the hypothermic range, 35° C equals 95° F. The threshold of severe hypothermia, 28° C, can be inverted to yield 82° F. Starting with these landmarks, an approximate 2:1 ratio between degrees Fahrenheit and degrees Celsius gives workable conversions. For example, 30° C, an increase of 2° from 28° C, is equivalent to 86° F, an increase of 4° from 82° F.

9 Describe the major causes of hypothermic death during rescue and resuscitation

Afterdrop is the continued cooling of a body after removal from cold. Due in part to conduction, it can be increased markedly with peripheral rewarming or muscular exertion, which rapidly return cold acidotic blood from the periphery to the core circulation.

Rewarming shock is almost universal in severe or prolonged hypothermia. Hypovolemia is caused by profound diuresis and fluid shifts out of the vasculature, worsened by sludging of cold thick blood, and augmented by cardiac collapse, vascular dilation, and loss of autoregulation. Afterdrop, peripheral rewarming, and exertion all challenge the collapsed circulation. Shock and death often result unless copious warmed IV fluids can be provided.

Asystole and ventricular fibrillation are common endpoints in severe hypothermia. Profound bradycardia and hypotension, often present initially, may be adequate to maintain the minimal metabolic needs of patients with core temperature below 28° C. Further cooling, rough handling, mechanical stress, and exertion have all been linked to deterioration from a perfusing to a nonperfusing rhythm.

10 What rules of advanced life support may be altered in hypothermia?

If the core temperature is known to be below 28° C, cardiopulmonary resuscitation (CPR) may be withheld in the presence of a pulse, no matter how slow or weak, and no matter the blood pressure. Some experts would withhold CPR in pulseless patients this cold if a narrow-complex QRS were seen on electrocardiography. In this temperature range, only minimal circulation is required, and CPR is relatively ineffective and may provoke ventricular fibrillation.

If, however, the temperature cannot be taken accurately, or if no pulse or narrow QRS can be established, CPR is indicated at standard rates. Earlier concerns about intubation have been put to rest by further study. Interventions should be made gently because cardiac arrest has been associated with rough handling, patient exertion, and iatrogenic cooling.

KEY POINTS: HYPOTHERMIA IN CHILDREN

1 Children are particularly prone to hypothermia, especially with serious illness or injury.

2 Serious hypothermia often goes undetected by clinicians without a high degree of suspicion.

3 Although it may provide for an occasional remarkable recovery after prolonged resuscitation, hypothermia is generally harmful.

4 Moderate and severe hypothermia alter standard resuscitation measures.

11 Should blood gases be corrected for patient temperature?

The long-standing belief that blood gases are best corrected for patient temperature has been overturned by evidence that uncorrected gases better reflect the true physiology of the hypothermic state, lead to more appropriate therapy, and are associated with improved outcomes.

12 What standard treatments are usually safe and effective in hypothermia?

Almost all hypothermic patients are volume depleted, and volume replacement with warmed isotonic IV fluid is essential. Administration of oxygen and maintenance of the airway is indicated. Although some patients will be hyperglycemic, hypoglycemia is common, and infusion of dextrose in 5–10% solutions is indicated if the blood glucose is unknown. Lidocaine and vasopressors may be helpful at times if indicated. Cardioversion of ventricular fibrillation may not be effective until after rewarming, but case reports of success at low temperatures probably warrant at least one or two attempts with standard energy settings before further rewarming.

American Heart Association: 2005 guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Part 10.4. Hypothermia. Circulation 112:136–138, 2005.

13 What commonly used treatments are not routinely recommended for hypothermia?

The routine administration of antibiotics or corticosteroids is not beneficial in hypothermia. Sodium bicarbonate is probably contraindicated, as acidosis is relatively physiologic in the hypothermic state, and the use of bicarbonate has provoked dangerous alkalosis after rewarming. Insulin for hyperglycemia is contraindicated, since this condition will usually resolve with rewarming, and the insulin will become effective only at that time. The same caution applies to many other medications.

14 Is room-temperature IV fluid thermally neutral?

At approximately 70° F (21° C), room temperature fluid is colder than all but the coldest hypothermia patients. The infusion of room-temperature fluid has been associated with further cooling of patients and with shock, ventricular fibrillation, arrest, and death.

15 What is the best way to deliver warm IV fluids in the ED?

Immersion-bath and coil-sleeve warmers do not effectively warm IV fluids, and they markedly slow flow. Microwave ovens can be used in a pinch to warm IV fluids in plastic bags. This requires prior experimentation to see how long a particular oven requires to warm a standard IV bag to a temperature of 40–44° C. Unless these fluids are given very rapidly through specifically designed short, large-bore IV tubing, however, they cool before reaching the patient. The preferred method is to use countercurrent blood warmers designed for rapid warming and rapid infusion. These come with large-bore plumbed tubing to ensure the delivery of warm fluid to the patient.

16 Describe three rewarming strategies

Passive rewarming involves placing the patient in a warm room, covering with dry (not warm) blankets, and allowing spontaneous gradual rewarming. This method has no demonstrated advantage in pediatrics, and is associated with increased morbidity and mortality.

Active external rewarming involves measures such as warm baths or blankets, plumbed pads, chemical heat packs, heat lamps, and forced-air warming. Although available in many settings, these techniques often promote the dumping of cold, acidotic blood from the periphery onto the core, and increase metabolic demand and circulatory work prior to volume expansion or cardiac rewarming. Thus, they are associated with afterdrop and rewarming shock, and may potentiate ventricular fibrillation. These methods should be reserved for stable patients with mild hypothermia. They are also useful to prevent cooling during the resuscitation of normothermic children.

Core rewarming methods range from the simplest, such as warmed IV fluids and heated, humidified oxygen; through warmed irrigation of the bladder, stomach, colon, peritoneum, or left pleural cavity; to extracorporeal circulation (i.e., the heart–lung pump). These methods are more effective and support the patient. They decrease the risk of afterdrop and rewarming shock. They are indicated in moderate to severe hypothermia; the more aggressive methods are preferred in the setting of hypothermia with circulatory compromise or cardiac arrest.

Walpoth BH, Walpoth-Aslan BN, Mattle HP, et al: Outcome of survivors of accidental deep hypothermia and circulatory arrest treated with extracorporeal rewarming. N Engl J Med 337:1500–1505, 1997.

17 If active external rewarming is sought, what is the best method?

Current evidence suggests that forced-air warming using devices such as the Bair Hugger (Arizant Healthcare, Eden Prairie, MN) may be most effective and associated with less afterdrop in patients with intact circulation. Such devices limit patient access to some degree. There is little evidence or logic to suggest that any external rewarming method would work well in patients with absent or severely depressed circulation.

18 What is the ideal rewarming rate?

There is no single ideal rate. Even if there were, achievable rewarming rates vary among patients with different degrees of metabolic and circulatory impairment. Rather, the method chosen should be matched to the patient’s needs and the severity of hypothermia. Rapid or profound cooling warrants rapid rewarming. Most children suffer from acute hypothermia and will tolerate rapid rewarming if given aggressive volume expansion and other support. The core methods above are listed in approximate order of increasing effectiveness. If the method chosen does not produce rapid rewarming, a more effective method should be employed.

19 Discuss the treatment of frostbite

After core rewarming to at least 35° C and after any risk of further chilling is past, frostbitten body parts should be rapidly rewarmed. This is done in a bath carefully maintained at 40° C to 42° C for 15–30 minutes, until flushing indicates reperfusion. Narcotic analgesia is indicated. Tetanus immunization should be updated, and the affected part should be elevated. Blisters that appear blue or black are left intact. Pink or white blisters should be opened because they contain thromboxane, which increases inflammation and promotes thrombosis. Either type of blister can be covered with aloe ointments. Prostaglandin inhibitors, such as ibuprofen, are administered orally to counteract the effects of thromboxane. Immediate amputation is never indicated. Admission and ongoing care are usually best directed by specialists in burn, plastic, or trauma surgery.

Koljonen V, Andersson, K, Mikkonen K, Vuola J: Frostbite injuries treated in the Helsinki area from 1995 to 2002. J Trauma 57:1315–1320, 2004.

Murphy JV, Banwell PE, Roberts AH, et al: Frostbite: Pathogenesis and treatment. J Trauma 48:171–178, 2000.

KEY POINTS: TREATMENT OF FROSTBITE

1 Rapidly rewarm frostbitten parts in a bath of water at 40–42° C.

2 Give narcotic analgesics.

3 Leave blue or black blisters intact.

4 Open pink or white blisters.

5 Give oral ibuprofen to counteract effects of thromboxane.

6 Admit the patient and involve surgical colleagues.

20 Does hypothermia alter usual prognostic indicators?

Yes. Intact survival has been reported after submersion for up to 79 minutes, immersion for hours, exposure for days, core temperatures as low as 14° C, and CPR for up to 9 hours.

Gilbert M, Busund R, Skagseth A, et al: Resuscitation from accidental hypothermia of 13.7 degrees C with circulatory arrest. Lancet 355:375–376, 2000.

21 Cite exceptions to the rule, “no one is dead until they’re warm and dead.”

This rule does not apply in the face of injuries incompatible with survival. It cannot be applied if the patient cannot be rescued from a cold environment in the first place, or if maximal available rewarming methods are ineffective and transport is not an option. Finally, failure to restore a circulating cardiac rhythm after 30 minutes of resuscitation above a core temperature of 32° C is usually a sign of irreversible death.

Auerbach PS: Some people are dead when they’re cold and dead. JAMA 264:1856–1857, 1990.

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